Before the music plays, before the first cocktail is served, before the models gingerly step onto the rooftop, designer Betsey Johnson is in an underground hotel space prepping for the evening show at which her swim collection will be unveiled.

By prepping, we mean planting a beauty mark on the face of each model, just east or west of the smile. The moment comes with a warning label: “The most important thing is I want you to have fun! You really can’t get it wrong. You want a glass of champagne?” she asks the models as she morphs from impromptu makeup artist into bartender. “This swim collection is all about fun.”

The Spring/Summer 2014 swim line, meant to bottle Johnson’s legendary quirk and endless energy, is a party collection of bows and peplums, of mesh and ruffles, built around retro, nautical and glam rock themes.

But before the show, Johnson held court in The Betsy’s down-under speakeasy, whose low-slung space — exuding an air the exact opposite of the touristy vibe outside — was serving as a styling salon for the Saturday night show. The sofas are now overflowing with Johnson’s trademark baubles, wigs and a rainbow of high-heeled stilettos and platforms. Oh, and there is a stash of licorice sticks and rosebuds that will become props too.

This is where Johnson seems most at home, on triple duty, engaging the questions of reporters, sipping champagne and vamping by way of the models walking by and her suggesting another layer of whimsy. “Try the thigh-highs. Roll them up even first, then roll them down kinda messy with that suit,” she implores before taking on the task herself. “Wait! Where is my personal collection of pinafores?” she says, grabbing a white one and tucking under a belt across the bottom of one of the models. Now the one-piece has a ruffled back, suited for the beach or the streets — at least in Betsey’s world, where the goal is to capture the imperfect nature of fun.

Second things second.

“Do you like my dress?” she asks, never really pausing for an answer. “It’s really not a dress. It’s a skirt! I just pulled it up and put on a belt.”

She loves the black skirt/dress. Skress? Loves the white skull prints, the simple yellow flip-flops and the blond extensions askew. The flip-flops even make the cut for her appearance at the show later that night, along with a fistful of necklaces.

Johnson, who turns 71 next month, started the label in 1978, building a brand that always winked at youthful rebellion, the octane of punk and rock-n-roll and the frill of femininity. By 2004, the label broadened as a lifestyle brand including handbags and luggage, footwear, bridal, kids, jewelry, eyewear and fragrances. Today, her collection can be found in more than 2,000 specialty boutiques along with wholesale ventures.

Johnson’s personality — she is known for doing a cartwheel and/or a split at the end of her runway shows — has also inspired a reality show, XOX Betsey Johnson, on the Style Network. Of course, she has been approached before but didn’t want the intrusion of 24/7 cameras. Johnson finally relented, deciding to try it with her co-star and daughter, Lulu Johnson, who also has an eponymous label. The show debuted in May.

“Ya know, I didn’t think I would ever get used to the camera’s being around all the time, but after a while, in some weird way, you forget they are there,” she says. “We were pretty open about my bankruptcy and my daughter’s divorce. I think fans enjoyed seeing another side of me.”

The eight-episode arc, which follows a tumultuous period for Johnson — from her filing for bankruptcy last year and closing her 63 retail stores to building her mother-daughter-relationship to Steve Madden taking over her company, where she is now creative director. The show was thick with Johnson’s energy but also chronicles her drive to reinvent herself at an age when others are retiring.

That comeback includes a new dress line (same DNA., lower price tag) available at brand-name department stores. She says she is now ready to take on other projects, too, including, eventually writing a book.

Now, back to Johnson’s reason for being in Miami. This was her first time at Mercedes-Benz Swim Week.

Rather than doing a runway show, Johnson unveiled the line at a launch party on the Betsy Hotel rooftop — hyped as meet Betsey at the Betsy — an event celebrating her new partnership with swimwear manufacturer Malibu Design Group.

The standout: a black one-piece called Betsey Meets Friend, featuring an iconic image of Marilyn Monroe that she describes as “kind of Lichtensteiny.”

“I wanted this to be a little bit of pretty girl and a little bit of rock-n-roll,’’ she says. “It is all about feeling sexy and comfortable. It is all about the fit and having lots and lots of fun.”

Third things third.

Before the show, Johnson was strolling down Collins Avenue and ran smack into a group of her fans.

“Here they were, telling me how happy they were to see me and how much a part of their lives I have been. These were women in their 40s,” she says, flinging her arms into the air as a sort of exclamation point. “They still remembered wearing my stuff when they were teenagers and into being an adult, and now too. They still remembered how much fun they had in that Betsey Johnson dress they wore for the prom. They still remembered the little girl in them. I wish them happiness forever and ever.”